1. Field of the Invention
The present invention relates to apparatus for dissolving between two different video signals.
2. History of the Prior Art
The ability to dissolve between two different video signals has become a very useful tool, particularly in providing special effects. For example, there are numerous situations in which a transition from one video signal to another video signal over a selected period of time is desired. A gradual transition in many such situations can have a much more desirable and pleasing effect than a substantially instantaneous transition. It is also desirable in certain situations to provide a partial dissolve in which portions of both video signals are present. This is particularly important in the special effects area where portions of two separate and independently derived images are superimposed to create a single image.
A number of different techniques are known for dissolving between different video signals. In one such technique the two different video signals are applied to the inputs of a video mix amplifier provided with a pair of handles for selectively attenuating the two different video signals. As the handles are manually manipulated a dissolve between the two different video signals is effected. This approach to dissolving is a relatively crude one at best. The rate of dissolve is dependent on the speed at which the handles are manually manipulated. Thus it is extremely difficult to dissolve at a constant, given rate, particularly where the rate is a relatively rapid one. It is virtually physically impossible to dissolve at the high rates demanded by some situations such as less than 1/10 of a second. Furthermore the difficulty or impossibility of dissolving over a selected number of frames as well as the inability to synchronize the dissolve with the frame intervals makes editing extremely difficult or impossible.
Other dissolver arrangements utilize time constant circuits or similar arrangements for providing a dissolve over a selected time interval. While some such arrangement go so far as to separate and utilize vertical information in their attempt to provide synchronism with the composite sync signal, such arrangement are incapable of insuring that the dissolve begins at the beginning of a particular frame and ends at the end of a particular frame. Such arrangement also suffer from other problems including a visually discernible coarseness during the dissolve and the inability to select the dissolve rate remotely.
Accordingly, it would be desirable to provide a digital dissolver which is closely synchronized with the composite sync signal.
It is a further object of the invention to provide a dissolver which separates frame pulses from the composite sync signal to that dissolve may be begun at the beginning of a given frame and terminated at the end of a given frame.
It is a further object of the invention to provide a dissolver in which the dissolve rate can be selected both locally and remotely and in terms of a specified number of frames.
It is a still further object of the invention to provide a dissolver capable of performing a variety of different functions in addition to dissolve between the two different video signals including the ability to stop at a predetermined point part way through the dissolve, the ability to reverse the dissolve after stopping part way through the dissolve and the ability to cut substantially instantaneously to the video source being dissolved to.